1. Field of Invention
The present invention relates generally to rotating disk type data storage devices. The method and apparatus of this invention has particular application to enhancing a disk""s data transfer rate.
2. Discussion of Prior Art
On a hard disk drive, data is stored in the magnetic coating of a disk. A transducer, usually called xe2x80x9cthe headxe2x80x9d, is held on an actuator arm and used to read and write data. A drive normally consists of multiple disks with each disk having a pair of associated heads, one head for the top surface and one head for the bottom surface. The disks rotate around their center at a constant rate, typically measured in RPMs.
Data is organized on the drive in cylinders, tracks, and sectors. Tracks are concentric rings on a disk and are similar in appearance to the annual rings of a tree. The number of tracks on a disk varies from one drive to another but ranges from in the tens to in the thousands. Tracks are typically numbered from the outermost to the innermost.
A cylinder refers to the collection of all similarly numbered tracks from all the disks. As disks are normally vertically stacked, the term xe2x80x9ccylinderxe2x80x9d is actually visually descriptive of the similarly numbered tracks.
Each track is further divided into sectors. The number of physical sectors per track was traditionally the same for every track; but now, most disk drives use zoned bit recording which places more physical sectors on the outer tracks of each disk.
The details of the physical layout of the hard disk drive are hidden from the computer system using a hard disk controller. The controller performs mapping between logical drive addresses and physical drive addresses. Two common mappings are logical sector addressing and logical block addressing. These mappings allow a computer to refer to data using logical addresses while the disk controller ensures the data is written to the correct physical location.
Traditionally, three types of mapping strategies have developed. Vertical mapping accesses data on an entire cylinder, starting from the top track and ending at the bottom track, before moving to the next cylinder. Horizontal mapping accesses data from the outer track to the inner track on a disk surface before switching to the next disk surface. Finally, combination mapping uses both vertical and horizontal strategies.
As previously mentioned, a head is used to read data from or write data to a disk. When data exceeds the storage capability of a single track, the data is split between multiple tracks. The time it takes for the head to move from one track to the next and to use embedded servo information to ensure it is correctly positioned for the next read is called read settle time. When writing data across multiple tracks a similar delay is present which is referred to as write settle time.
The problem created by these settle times hinders disk performance by reducing the sustained data transfer rate. As an example, for a track having 16 sectors, data read from track 1 starts at sector 1 and proceeds sequentially to sector 16, the last sector in the track. Reading then continues on track 2 at sector 1. However, during the read settle time, the disk keeps spinning and the first few sectors (of track 2) actually rotate past the head. By the time the head is capable of reading data, the sector beneath it is no longer sector 1. Consequently, the drive must wait for the disk to continue rotating until sector 1 is underneath the head again.
The above problem is avoided by track skew which offsets the starting sectors on adjacent tracks to minimize the wait for the first sector when switching tracks.
Similarly, problems caused by write settle time may also be addressed with track skew; noting, however, that write settle time frequently exceeds read settle time and requires a greater skew value. Determining a value for track skew, therefore, has historically been a compromise and does not optimize a disk drive""s sustained data rate for either reading or writing.
Whatever the precise merits and features of the prior art in this field, the earlier art does not achieve or fulfill the purposes of the present invention. A track skew value optimized for a smaller read settle time is not provided for in the prior art, and neither is a logical write skew, different than the read skew, which is used when writing to the disk.
A disk drive""s logical track layout uses a skew needed to compensate for a read settle time. Also, the LBA numbering scheme for the drive follows this logical track layout. Read operations, therefore, proceed exactly as expected, similar to present hard drives. On writes, however, the drive controller switches to a xe2x80x9cwrite logical trackxe2x80x9d layout which uses an effective skew that is optimized for a write settle time. Doing so necessitates buffering write data and writing logical blocks to the drive out of sequence.